When an engine is controlled, a property of heaviness and lightness of fuel may not sufficiently match an engine control characteristic. In this condition, an air-fuel ratio may not be properly controlled. Consequently, air-fuel mixture may not be ignited at a desired ignition timing, and drivability and emission may be degraded. When the property of heaviness and lightness of fuel changes, the specific gravity and volatility of fuel also changes. As a result, combustion efficiency and exhaust components change, according to the heaviness and lightness of fuel. In particular, at the time of starting of an engine, when gasoline becomes heavy, volatility of the gasoline decreases. In this condition, a quantity of fuel adhering to an inside wall of an intake pipe increases in a port injection engine. Alternatively, a quantity of fuel adhering to an inside wall of a cylinder increases in a direct injection engine. Thus, air-fuel mixture becomes lean, and an air-fuel ratio increases. Contrarily, when the gasoline becomes light, volatility of the gasoline increases, and the adhering quantity of the fuel decreases. Thus, air-fuel mixture becomes rich, and an air-fuel ratio decreases.
According to JP-A-3-179150, for example, the property of heaviness and lightness of fuel is reflected to an injection control so as to restrict degradation of drivability and emission. Specifically, in JP-A-3-179150, the property of heaviness and lightness of fuel is detected by using a heaviness/lightness detection sensor provided in a fuel supply pipe or a fuel tank, and an injection quantity is corrected according to the detected property of the heaviness and lightness. When fuel is heavy fuel, an engine output becomes insufficient. Therefore, the injection quantity is corrected and increased when the fuel is heavy fuel, so that engine torque is increased, and the engine rotation speed is gradually increased. Here, a fuel injection quantity is corrected and increased in a specific startup period before warming up of the engine. When the period is set longer than an appropriate length, emission may be degraded. JP-A-3-179150, does not necessarily appropriately determine the period, in which the fuel injection quantity is corrected and increased, and may not steadily produce a preferable emission characteristic.
A conventional fuel property determination apparatus of an internal combustion engine, for example, determines a property of fuel to be heavy when rotation speed of the engine is lowered by a specific value from a target rotation speed during a specific period after starting of the engine. Moreover, the conventional fuel property determination apparatus also determines fuel to be heavy when an integrated value of rotation speed of the engine during a specific cycle after starting of the engine is less than a specific value.
However, friction or the like of the engine may smooth and reduce variations in rotation speed of the engine caused by the difference in properties of fuel. Therefore, the property of fuel cannot be determined with high accuracy on the basis of the rotation speed of the engine. Further, it is difficult to detect the heaviness of fuel with high accuracy and hence it is difficult to determine intermediate fuel having an intermediate property between those of the light fuel and the heavy fuel.
For example, JP-B2-7-50098 proposes an apparatus that calculates a combustion rate on the basis of combustion pressure of an engine and compares the combustion rate with a threshold, which corresponds to combustion rate of base fuel, to determine the property of fuel.
In general, an ignition timing of an engine greatly contributes to a state of combustion such as the combustion rate and the burning period, and when the ignition timing varies, the combustion rate also varies due to variation in the state of combustion. Accordingly, in determining the property of fuel in accordance with the combustion rate similarly to JP-B2-7-50098, a false determination may be made because of variations in the combustion rate caused by variations in the ignition timing.
In view of the foregoing and other problems, it is an object of the present invention to produce an engine control apparatus and a fuel property detection apparatus being capable of maintaining a state of combustion in starting of an engine. It is another object of the present invention to produce a fuel property detection apparatus and a fuel property determination apparatus each being capable of accurately determining a property of fuel.
It is another object of the present invention to produce a method for controlling the engine and methods for detecting and determining a fuel property.